W Dressler r. riedel Table 4. Thermo-mechanical properties of SiC-ceramics Property SSic HPSiC HIPSiC LPSSiC Density(g cm 15 3·21 Porosity(%o) Youngs modulus(GPa) 410 Coefficient of thermal 4·5 4·5 expansion(30-1500°C 10°K Thermal conductivity at 600C Fracture toughness(MPa'm 25-6:5 30-40 30-70 Flexural strength(MPa) 730 400°C Hardness(GPa) 60 Oxidation rate constants 10-6-11·2 107-11·1 kg"(m s) 1400°C 1370°C Critical thermal shock temperature(C: ATc) develops during sintering of B-SiC containing starting powder due to the occurrence of the B/a-transformation. Padture reported the onset of transformation accompanied by the platelet a grain growth to be at 1950 C 42 The fracture behavior of liquid phase sintered Sic-materials is unequivocally different from the ceramics sin- tered with boron and carbon. It has been ER shown4, 120, 120, 142-14 that the cracks in liquid phase sintered SiC ceramics propagate mainly intergranular opening the possibility to influ ence the fracture toughness by microstructure design as it is well known for SiN, ceramics. Figure 11 reveals the influence of B-Sic con tents on the microstructure of sic densified by dding 10.42 wt%Y2O3 and 2 98 wt% ALOx Using pure a-SiC powder an equiaxed homo geneous microstructure is obtained showing no exaggerated grain growth as observed in B, C b sintered SiC, while sintering of nearly pure B-SiC leads to the formation of platelet-like a-SiC crystals having a narrow grain tribut The influence of these different microstructures on the fracture toughness of the received SiC ceramics is pointed out in Fig 12. 6 This in-situ toughening has also been observed by Padture 5 and Lee+ I who showed crack bridging and grain pullout to be the main reinforcing mechanisms. Additionally, the strength of pressureless sintered SiC can be improved from 430 to 730 MPa by liquid phase sintering(Table 4). Thus, the strength Fig. 11. Microstructures of liquid phase sintered(10-42 wt% Y,03+2 98 wt% Al,O,(SiC-ceramics(a)a-SIC and fracture toughness can be improved by tai- starting powder(UF-15, Lonza AG) ;(b)p-Sic-starting loring the microstructure similar to si,n, cera- powder(UF-15, Lonza AG)26 W. Dressier, R. Riedel Table 4. Thermo-mechanical properties of SiC-ceramics'"" 137 1411 Property SSiC HPSiC HIPSiC LPSSiC Density (g cm -~) 3.15 3.20 3.21 3.21 Porosity (%) <2 0 0 < 1 Young's modulus (GPa) 410 450 450 420 Coefficient of thermal 4.9 4.5 4.5 4.5 expansion (30-1500°C) (10 " K ') Thermal conductivity at 600°C 50 55 75 50 [W (mK) '1 Fracture toughness (MPa. m '/2) 2.5-6.5 3.0-4.0 -- 3.0-7.0 Flexural strength (MPa) RT 430 640 640 730 1400°C 450 650 610 400 Hardness (GPa) 28 32.7 40.4 26 Oxidation rate constants 10.6-11.2 10.7-11.1 -- 8.5-9.7 [kg 2 (m 4 s) '1 ( - log kp) 1400°C 1400°C 1370°C Critical thermal shock 380 -- -- -- temperature (°C: ATc) develops during sintering of fl-SiC containing starting powder due to the occurrence of the fl/a-transformation. Padture reported the onset of transformation accompanied by the platelet grain growth to be at 1950°C. '42 The fracture behavior of liquid phase sintered SiC-materials is unequivocally different from the ceramics sin￾tered with boron and carbon. It has been shown 14"12°'126"142-146 that the cracks in liquid phase sintered SiC ceramics propagate mainly intergranular opening the possibility to influ￾ence the fracture toughness by microstructure design as it is well known for Si3N4 ceramics. Figure 11 '~ reveals the influence of fl-SiC con￾tents on the microstructure of SiC densified by adding 10.42 wt% Y203 and 2.98 wt% ALOe. Using pure a-SiC powder an equiaxed homo￾geneous microstructure is obtained showing no exaggerated grain growth as observed in B, C sintered SiC, while sintering of nearly pure fl-SiC leads to the formation of platelet-like a-SiC crystals having a narrow grain size dis￾tribution. The influence of these different microstructures on the fracture toughness of the received SiC ceramics is pointed out in Fig. 12. '6 This in-situ toughening has also been observed by Padture' s and Lee' "~" ' 7 who showed crack bridging and grain pullout to be the main reinforcing mechanisms. Additionally, the strength of pressureless sintered SiC can be improved from 430 to 730MPa j4" by liquid phase sintering (Table 4). Thus, the strength and fracture toughness can be improved by tai￾loring the microstructure similar to SLN4 cera￾t 5p.m i Fig. 11. Microstructures of liquid phase sintered (10.42 wt% Y20~+2"98 wt% A1203 (SiC-ceramics. (a) ~-SiC￾starting powder (UF-15, Lonza AG); (b) fl-SiC-starting powder (UF-15, Lonza AG).